text:
How much water pressure can flight recorders withstand?
How deep can black boxes go under water and how much pressure can they withstand while under water before they get crushed?
I'm a layman in terms of aviation, so if it's not too much to ask, can you answer as easily understandable as you can?

How deep can black boxes go under water?

They are required to be able to withstand a depth of 20,000 feet (3.8 miles, 6.1 km) for 30 days.

& how much pressure can they withstand?

At 20,000 feet deep the pressure is 8900 pounds per square inch (psi) which is 606 times atmospheric pressure at sea level (nearly 4½ tons on each square inch of surface)

A flight data recorder from AF447, around 13,000 feet deep for almost 2 years. Still readable

References

The DFDR must be watertight to a depth of 20,000 feet in sea water, and survive at this depth for 30 days
The units also have to withstand a static crushing force at all of its six axis points of an applied load force of 5,000lbs for 5 minutes on each axis.

See: What happens to the transponder, FDR and CVR in case of an explosion?

The southern corridor of the Indian Ocean, where officials now believe the plane went down, ranges in depth from 3,770 feet to 23,000 feet.  At its greatest depth, that’s almost equal to the height of Mount Everest (which measures 29,029 feet tall).

See: Indian Ocean depth proves great challenge for MH370 search crews

For every 33 feet (10.06 meters) you go down, the pressure increases by 14.5 psi (1 bar).

See also

List of unrecovered flight recorders

"8900 pounds per square inch" Or approximately 626 kg/cm^2 (which doesn't mix metric and imperial units), if my conversion is accurate. Like having the full weight of a car pressing down on a surface of area 2-3 cm^2.


text:
Does a safety pilot for a PPL practicing under the hood in VFR conditions need a current medical?
Am I legal to fly under the hood with a safety pilot who does not have a current medical? VMC only, not IMC.

As explained in AC 68-1A, a safety pilot requires a current medical:

A pilot acting as a safety pilot per part 91, § 91.109(c), however,
  would be a required pilot flightcrew member by regulation rather than
  a passenger. Section 91.109(c) provides that no person may operate a
  civil aircraft in simulated instrument flight unless the other control
  seat is occupied by a safety pilot who possesses at least a private
  pilot certificate with category and class ratings appropriate to the
  aircraft being flown. Because a safety pilot is a required pilot
  flightcrew member, the safety pilot is required to hold a valid and
  appropriate medical certificate in accordance with § 61.3(a).

For information on a safety pilot using BasicMed see this question.
By the way, by definition a safety pilot is only required in VMC. In IMC you're in actual instrument conditions; see this question.

Thank you, especially the FAR reference.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Is there a specific purpose for the silver "swoosh" on Air Force One?
Air Force One, just ahead of where the wing joins, has a silver "swoosh" on the bottom of the fuselage:

Air Force Two, or other congressional aircraft painted with the blue/white livery that I was able to find do not have that same feature:

What is the purpose of this "Silver Swoosh"? I was bale to identify the following two paths of reasoning, which raise additional questions:
Option 1: Part of the Paint Job

If the silver is just part of the paint job, why don't other congressional aircraft have that same feature?
If it is part of the paint job, what does it signify or what drives its inclusion?

Option 2: It fulfills a defensive function

What defensive purpose does it fulfill?
Since it seems to be highly reflective, does it potentially serve as a mirror for missiles with certain seeker heads?

It's just for looks; an area of unpainted aluminum, polished.
In this close up you can make out the rivet lines and skin laps of the bare aluminum surface.  With a little work it's easy to get a finish like chrome plate, especially on a queen of the sky like that.

Back during American's natural finish paint scheme days, 80% of the fuselage was unpainted and polished.  AF1 just does it on a little patch so it's like a chrome accent.
There was significant weight saving in AA's case, but they finally gave in and painted their entire airplanes because the polishing has to be done regularly and is a pain, and over time the polishing, on an airplane that spends a lot of time outside and constantly re-oxidizes, slowly removes the pure aluminum cladding of the metal that is its corrosion protection.
Vinyl wrap is now starting to become common on airliners, and in theory you could probably do that sort of accent today with "chrome look" vinyl wrap, but AF1 has had that paint job for decades.

They are the queens of the Air Force ^^

Not doubting you, just curious on your source for this. As it stands, this is "a guy on the internet said..."

My only source is "been in aviation for 40 years and it's obvious".  But you have a point, so I added a close up and some context.

I used to really like the way those "natural" AA planes looked.

Plain aluminium was common on freighters..The Flying Tigers and Northwest B747Fs didnt have much paint on them. They were shiny ex-factory but the planes were too busy flying and they looked pretty rough after a few years.

Some classic airplane owners and homebuilders do their airplanes in natural finish for that mirror look that draws ppl at shows, but then they discover how much work is required to maintain the finish, even if hangared, and later on paint them.

As a follow up to this, it is just part of the paint/decoration. Polished aluminum.
This pic is directly over my house. They use the local airport for training, on a monthly basis.

(Image taken by me, directly over my house.)

At first I thought this adds nothing new to the older answer, but actually the picture attached does convincingly display the whole aesthetic composition the "swoosh" is a part of. +1 for that.

Agree with @Jpe61 ... picture worth 1,000 words, this is to me the more convincing answer.

The 747-based Air Force One (VC-25A) uses the livery that was first used on the 707-based Air Force One (VC-137) shown below (wiki commons photo).

The polished aluminum area that you observed on the 747-based Air Force One, appearing as a "swoosh", can be seen as it was originally designed for the 707 AF1 (VC-137), with the polished area more clearly continuing all the way to the tail of the aircraft.
On the 747 AF1 the polished aluminum is interrupted by the painted aerodynamic wing-root fairing, making it appear as a "swoosh".
As a result of aerodynamics expertise increasing toward the end of the 1960s when the Boeing 747 was being designed, Boeing designed an aerodynamic fairing to smooth the turbulent airflow at the wing root where the wings attach to the body.
During the development of the 757 and 767 in the very late 1970s Boeing decided to utilize composite materials in order to save weight on light aerodynamic components like flight controls and fairings...  eventually Boeing applied this weight saving technology to the wing-to-body fairing of the 747.
Because composite components cannot be polished to a shiny luster like aluminum can, Boeing paints these parts grey.
The "swoosh" on the 747 AF1 is a result of the polished area (which is uninterrupted on the 707 AF1) being interrupted by the painted composite wing-to-body fairing.
Regarding the 757 (C-32A) livery which lacks the polished metal aspect of the 747, it may have been omitted due to the C-32 not having the specific primary role of transporting the President.
The C-32 is intended to carry the Vice President, the first lady, and the secretary of state. Other members of the president's Cabinet and members of Congress have flown aboard the C-32A, as has the president when traveling on short domestic trips where the range or capacity of the 747 is not required or the runway is not long enough to accommodate a 747.
Perhaps the most likely explanation for the 757 not having a polished metal area is that it takes too much manpower to keep the bare metal areas of the two 747s polished and looking good for the cameras.

this is purely a joke but it is likely because of the Nike Air Force One brand because there is no Nike Air Force 2 brand so the only one with a swoosh is Air Force One as it was the only one designed by Nike.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is this aircraft from ca. 1925 (crash included)?
I recently found the following family pictures from ca. 1925 and have been trying to identify the aircraft in them:

A second picture of the same aircraft was taken shortly after the first one:

Both of the occupants supposedly walked away unharmed - my guess is that confident look in the first image didn't linger.
Judging by the inscription on the top of the second image they were taken in Lausanne, Switzerland.
Bonus Question: Does the registration number of 85 mean anything in particular? For example, is this the 85th aircraft registered in Switzerland? Or the 85th aircraft of this type made by the manufacturer?

http://www.airhistory.org.uk/gy/reg_CH-.html lists 85 as a DFW C.V Halberstadt - suggesting a Halberstadt C.V built by DFW.
Halberstadt C.Vs were built by Halberstadt, Aviatik, BFW and DFW and possibly others so this fits.
The 85 refers to the number it was registered as (CH-85), in Switzerland, which was probably assigned in sequence but may not have been.
A single example of this type of aircraft survives, in Belgium.


text:
What is my BHP if I open full throttle and how is it controlled?
In my Cessna 152 POH it is written that the horsepower rating is 110BHP at 2550RPM. My question is: is it 100% of the power of my engine or, as someone told me, if I open full throttle I will get 75% BHP? If so, then why don't I get 100% BHP at 2550 RPM? And is there any relationship between the mixture setting and BHP?

This is usually measured at sea level on a standard day, so if you are flying at a low altitude field on a standard day, you can expect that the engine is developing 110BHP at 2550 RPM. Mixture absolutely does affect power. Its not like a car where some of the HP is lost in the drivetrain.

Thank you for the ans. But my question is if I use recommended lean mixture setting, will it reduce the BHP? In performance chart of the POH it is given, at recommend lean mixture setting and standard temp at 2000ft at 2400RPM my engine will produce 75%BHP. At 2400RPM the engine is supposed to produce more BHP. The what is the reason behind it?

2400 RPM is less than 2550 where the engine was rated, and you lean the mixture to match the amount of air (oxygen) present in the intake air. If the mixture is too rich or too lean you will lose power. Usually best power is made right around peak EGT, but I'm guessing your 152 doesn't have an EGT so usually the procedure is to lean until it gets a little rough then 2 turns back in, right? You make less power with more altitude with reduced oxygen so your engine produces less power, you need to lean to match to make the best power at that altitude.

But to put it easy, no, using the recommended lean will not reduce power, it should increase it.

I understood. But why the RPM is more and the BHP is only 75%? If I consider 2500RPM =110BHP then at 2400 it should be 105.6BHP which is 96% of total BHP, then what is reason for which I am getting only 75% BHP?

I'll post an answer in a minute detailing this.

Rated BHP is measured at sea-level on a standard day. For the Cessna 152 I'm guessing you have an O235-L2 which is rated at 110HP @ 2550 RPM.
Now to understand how mixture affects the power produced by the engine you need to understand that the mixture knob controls how much fuel enters the cylinders to be burned with the air. As a combustion engine sparks, the fuel ignites but there is the fire triangle, Air, Fuel, Heat. Too much or too little of any of those pieces means a reduction of power or no power at all.
No, depending on the fuel type there is an optimum stoichiometric ratio where the proper amount of air and fuel are burned such that there is no extra air or fuel left over. For gasoline engines this is about 15:1, so for every 15 units of air, 1 unit of fuel is required. Having a richer fuel mixture results in an incomplete burn and may cool the cylinder temperatures, reducing power.
On the flip side, having too little fuel for the air can cause hotter cylinder temperatures and hot-spots on the cylinders. This causes detonation or knocking and can damage an engine very quickly. This is especially apparent in engines that have a lot of cylinder carbon deposits where the carbon can heat up and detonate the mixture before the spark plug does. Its like hitting the piston with a sledge hammer as its still coming up.
I don't have a graph from an O-235L but I did find one from an O-235C, which shows the relationship between throttle and BHP:

So you can see that reducing power from 2500 RPM to 2400 RPM gives a 10% decrease in BHP produced by the engine. Running at 2350 RPM is a 75% power setting (and in this graph nicely matches up with ~75 BHP). You can also see that there is a sweet-spot with relation to fuel consumption and produced power right around the 75% mark, which is usually refered to as the "performance cruise" value (best speed for fuel), and at 65% for "economy cruise" (best fuel).
So the mixture control needs to be leaned out based on the amount of air entering the engine. The levels of oxygen decrease with altitude and therefore the mixture needs to be leaned out. Its not uncommon for operations out of high altitude fields to perform take-offs with the mixture leaned, especially on hot days.
So its important to remember that the power produced by the engine is not a straight linear calculation. When you did your comment:

But why the RPM is more and the BHP is only 75%? If I consider 2500RPM =110BHP then at 2400 it should be 105.6BHP which is 96% of total BHP

You assumed that the engine can produce power at 10RPM, which just isn't true. Lets take a better example and say that at 1600 RPM it is producing 25HP and at 2600 it produces 100HP, we can do the math then.
Using a linear interpolation formula this gives us 85HP which the graph tells us is 80 HP. Remember though, the graph is curved, and linear interpolation is, well, linear.

It cleared many of my doubts. You just probably missed I wanted to know, if i lean the mixture will it increase/decrease my BHP?

The answer is "yes". But enriching it can also decrease or increase power,  the take-away is that you adjust the mixture for best power, sometimes that means leaning it,  other times you need to enrich it.

How do they measure the rated BHP on the ground? Tie the plane to a fixed tether with a load gauge?

No, they use an engine dyno.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Is the "effective wing span" reduced in a turn?
The Ace the technical pilot interview book says that in a bank the aircraft's lift is reduced due to an effective reduction in wing span. I know in a bank the vertical component of the lift vector is reduced due to the inclination of another component of lift towards the banked side. So the overall lift required to counter the weight is increased. That's why we need to increase the AOA or speed to maintain a level turn. But I am a bit confused about how the effective wing span is reduced, can anyone explain?

Your understanding is correct.
"reduced effective wing span" is just an analogy to help some people understand the concept of lift in a turn. The Total Lift is not reduced, it is just applied in a different vector in relation to gravity. The vertical lift component (in relation to gravity) is reduced and you can then imagine that the effective wingspan is also reduced by the same amount.
Banked turn (from Wikipedia)

That's kind of a strange way to view the problem of decreased vertical lift in a turn, but essentially it's true.  If you look at an airplane in a bank of angle theta, and imagine the tip-to-tip span of the wings as the hypotenuse of a right triangle, then the base of the triangle, which is the length of the hypotenuse multiplied by the cosine of theta will be shorter than the span is.
But that is an odd way to consider that problem.

Well, in a certain frame that makes sense. If you look at the airplane from the ground, in a bank, you will see a wing span of
$$\text{Wing Span} \times cos(\theta)$$
This is equivalent to the vector force that is applied in the vertical direction*
$$\text{Lift} \times cos(\theta)$$
This is not the same thing, but they are sort of pseudo-related quantities where all of the factors that affect each quantity align very conveniently.

*As an aside, this is where you get those tables for G-load in a turn, which ends up being $${1 \over cos(\theta)}$$

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What is NH overspeed test and how it is preformed on DHC Q400 aircraft?
In the Aeroplane Operating Manual of DHC Q400 aircraft it is written that, at the time of normal engine shutdown, the engine system tests the NH overspeed circuitry by using it to shutdown the engine.

I didn’t understand that, how the NH O/S circuit is used to shutdown the engine and how it is tested?
And what are the conditions that will ensure the correct operation of NH O/S circuit?

The same statement is made here in a document describing the PW150 turboprop engine, which powers the de Haviland Canada, now Bombardier,  Dash-8.
It gives a little more detail in section 12.23.14.4, NH overspeed protection.
It says:

The PW150A Powerplant has an independent overspeed (O/S) protection circuitry (dual chan- nel) built into the FADEC which has the capability to cut off the fuel flow through the Fuel Shutoff Solenoid. Independent NH signals (from the FADEC) are used by the O/S circuitry. A fuel shutoff command is issued when the measured frequency of the NH input signals exceed a pre-pro- grammed threshold value of 108%. The O/S Protection circuitry is exercised on normal shut- downs by the FADEC.

So, it would appear that, when testing the overspeed protection in a normal shutdown, they do not actually allow the shaft speed to increase to 108%, but rather they electronically alter the speed sensor frequency signal, so it appears like the shaft speed is greater than 108%, which then cuts off the fuel flow.
NH is the term used to describe the high pressure shaft speed, measured in units such RPM.

Thank you for the reference.. I read that paragraph just now.. At first I thought that, when the frequency of the electrical signal is 108% of it’s value then the solenoid will close fuel flow. But it is clear now that, when the NH RPM is 108% of it’s value, then it will alter the frequency and FADEC will close the fuel flow through solenoid.  But how it is used to shutdown the engine normally and teated for correct operation? I mean, to test it’s operation the NH RPM will have to reach 108% of it’s value. Does it happen in real time operation?

@Mirajul.Pias. If the aircraft is flying along, and NH exceeds 108%, the O/S protection cuts off the fuel flow. But when the O/S protection is being tested in a normal shutdown, the NH speed does NOT reach 108%. Instead, the bias the electronic speed sensor signal so it only looks like the NH speed has reached 108%. I modified the answer with the text in bold, as it might have been a little ambiguous. Hope that makes it crystal clear.


text:
Can jet engines be used for VTOL civilian aircraft?
Why do almost all civilian VTOL aircraft existing today use propellers (or rotors) for take off and landing and not jet engines (as the Harrier does)? Is there a technical reason for this?

The V22 Osprey is a good example of a VTOL propeller aircraft.I guess you might also be able to include helicopters too ;-)

the V-22 Osprey would spring to mind @kepler22b, that and the Convair XFY-1 Pogo

No I actually mean VTOL with specific focus on civilian aircraft not military ones. Thanks.

You really should edit the question to make it clear you mean civilian aircraft. (But the answer will be that there aren't any due to cost, reliability and possibly high level of pilot training required.)

Ca you name a purely civilian VTOL aircraft that uses any type of engine?

@M.Amin you should edit your question to clarify you're asking about civilian aircraft. Also, why are you talking about T/O and landing? Are you asking for VTOL only (like helicopters) or STOVL / VSTOL planes?

@Jamiec The AgustaWestland AW609 is a tilt-rotor (so a VSTOL aircraft) designed for civilian purposes.

To be more specific, let's take for example the new XTI Trifan 600 Aircraft,  is it possible to have the same design but based on jet engines?

@MarcoSanfilippo - so we have a sample size of 1 (with rotors) and we're asking why there are no jets? Answer: None have been designed/developed yet.

Not civilian, but the Do-31 has been the only VTOL transport jet built so far. https://www.youtube.com/watch?v=x3YueCf1JeI

Why was this question held as "unclear", it's perfectly clear, and has been answered exactly... Do I miss something?

Can?
Of course they can.
Does it make sense?
No, it does not.
A VTOL needs thrust at least equal to its weight. That is much more than a fixed-wing aircraft (HTOL) needs. Airliners have lift-to-drag ratio around 18–20. That is in clean configuration; in landing configuration it might be about half (~10) and we need twice as much thrust in case an engine fails, so around ¼ weight. So a VTOL would need 4 times as much thrust as a HTOL.
Likely more, because the craft would still have to be capable of reasonably controlled landing in case of engine failure. And while rotorcraft can do that by autorotation and pilot of Harrier or F-35B can simply eject, jet-powered civilian VTOL would only have the remaining engines for it.
Now fortunately thrust and power are only indirectly related. To create thrust, force must be applied to the air. This force accelerates air, giving it some momentum and some kinetic energy. Thrust is (change of) momentum per time and momentum is mass time velocity while power is (change of) energy per time and energy is ½ mass times speed squared.
So we can produce a lot of thrust with little power by accelerating a lot of air by a little. Which means large propeller: a rotor. That's how helicopters get enough lift/thrust with reasonably small engine.
Unfortunately rotors are not very practical for flying fast. One would also be rather difficult to tilt. That's why V-22 Osprey and Augusta-Westland AW603 use a compromise that is somewhere between; large enough to still have good efficiency in hover and small enough to be possible to tilt.
What would definitely not be efficient for hover is a jet though. Jets are small and can't affect that much air; what they can is accelerate it a lot. That is necessary for flying fast, but it is terribly inefficient when flying slowly.
Military jets don't care. They want so powerful engine to fly very fast anyway and they don't care for efficiency much as decisive edge in combat is worth a lot of money. But for civilian transport the small advantage of being able to land closer to destination would not outweigh the disadvantage of being several times more expensive to operate. Flying the long distance with HTOL to a nearby airport and, if the destination is not reachable by car, hiring a helicopter just for the final part of the trip ends up being a lot cheaper.

Thank-you very much Jan for the detailed answer. This actually was the perspective I was looking for.  Do you recommend any readings or books about this topic?

Jet propulsion is impractical for civilian VTOL, because the jet blast quickly erodes even a reinforced concrete landing pad, nevermind an asphalt driveway or a grassy field.  Military installations can afford special pads, but that infrastructure cost would be daunting for an aircraft whose advantage is that it's supposed to land "anywhere."
Also, takeoff and landing would be objectionably loud: 125 dB at 100 feet away for the Harrier, for instance.

The Wikipedia page on VTOL has a whole section on the history of "jet lift" VTOL aircraft. It includes some pictures of the Harrier "Jump Jet", which is probably the most widely recognisable VTOL fixed-wing, and has a jet engine. I think the only reason "almost all" VTOL aircraft use propellors is that almost all VTOL aircraft are helicopters. VTOL fixed-wing aircraft turn out to be quite hard (read: expensive) to get right and not many designs (with props or jets) have made it into production.

Jet engines have already been used for VTOL aircraft, see the Harrier or the Joint Strike Fighter.

Yeah, but is there any aircraft in the civilian space which uses jet engines for VTOL operations? Thanks.

There is at least one civilian, jet-powered, thrust-hover aircraft: the Zapata Flyboard Air.  This uses tiny (model aircraft size) turbojets to provide thrust for hover flight.  It has good safety margins, can fly with one engine out and land safely with two down.
It is completely impractical; it's a toy.
The only other civilian aircraft remotely in this category to date is the various iterations (over the past fifty-five or sixty years) of the Moller Aircar (it's had various names).  It's a ducted fan thrust hover craft that started out in the early 1960s looking like a tiny flying saucer and ended, without ever seeing production, around 2000 as a computer-controlled VTOL with transition capability and a purported top speed of 200 mph (though the prototype never got beyond tethered hover, as far as I'm aware).
Getting such a machine certificated with piston (or, for the last couple versions, rotary) engines would have been a nightmare; those engines, at the needed power and weight levels, can't be made highly reliable, which means the FAA won't let them carry people outside the Experimental category.
There was a brief effort to make the Williams "Flying Garbage Can" into a civilian craft; that never went beyond repainting a military prototype in a color other than olive drab -- same issue, reliability, as well as the fuel consumption of an engine several times the size of what's needed to fly with lifting surfaces.
A couple "jet pack" type devices have been built as one-off machines, but again, there's no practical application and they're too expensive and hazardous to make good toys.


text:
Does mid-air refueling have very hard constraints on weather?
I'm guessing mid-air refueling has constraints on windspeed and wind direction, visibility, and precipitation. I may have missed some, but those alone make me feel like mid-air refueling can't be done most of the time (?)
About wind direction though, I guess you could just fly in the "right direction", but then this would also have constraints because the "right" direction could lead away from your ultimate destination.
(also, is there a mid-air refueling tag I couldn't find?)

Air refueling is done up high most of the time - if you look up AR tracks, most are above 20,000 feet. Other than thunderstorms, which you should avoid anyway, there's not much up there. And we already have tools  to predict winds aloft, though it really doesn't matter as long as both aircraft are flying in the same wind as it will affect them roughly equally

@SSumner I would have thought turbulence could occur anywhere. I could have sworn I experienced turbulence on long commercial flights which regularly go to at least 30,000 ft.

I didn't mean to imply turbulence doesn't exist up that high, just that it is less common. Plus there are decent weather prediction tools that can let you estimate where said turbulence will be.

The receiver has to have visual contact with the tanker to complete the rejoin, so finding a clear block of airspace (i.e. not IMC) is necessary. Beyond that, none of the factors mentioned really matter much.  Turbulence can make refueling more difficult, although it's often possible to find an altitude with a decent ride.
Mostly, AR can be done; the times when it can't are much more the exception.

I think given that the exception tends to be planes dropping out of the sky, pilots try extra hard (there is usually no safe fail option).

Sure there is... divert. Not good for the mission, but given that AR is not a sure thing, the fuel planning is done so that for each AR, it's known ahead of time: if we can't AR, then we divert here.  In some instances (used more fuel over the target than planned), that's not possible, but for most refuelings, the alternative to a good AR is a divert, rather than a splash.

The visibility limit is probably less than VMC. The aircraft that wants to refuel can be guided to the tanker using TACAN to well below 1 nmi provided there are no other aircraft around.


text:
Is the stall speed different at -1G?
This question makes me wonder what happens at -1G.
When the load factor of an aircraft increases above 1, the stall speed increases. As the load factor comes closer to 0, stall speed decreases and becomes "infinitely" low (basically 0 kts) as an aircraft can not stall at 0G. But what happens at a negative load factor?
For example, during inverted flight the airfoil is still producing lift when the angle of attack is negative (enough).  Thus the stall speed should change and as the efficiency of a inverted wing is (almost definitely) not as high, I am assuming that the stall speed decreases.
But is a negative G maneuver necessarily to be done in inverted flight? No!
As a glider pilot,  I can refer to negative Gs during winch launches, or rather when they fail. You often practice cable fails, where your instructor disconnects you from the winch. To maintain speed and prevent a stall you push the nose down, ideally flying a small Zero-G parabola (as this will prevent a stall). But what if you push even more? Where is the stall speed going to be at?

It is more correct to reference stall angle of attack than stall airspeed.  A given airfoil can technically stall at any airspeed once AoA increases beyond the critical AoA.

The reason I am referring to the speed more then to the AoA is because the speed is what changes with the load factor, and thats what I am interested in.

AoA changes with the load factor as well.  In fact, I believe AoA will instantaneously change with load factor.

Well that is some new information for me, thanks for sharing!!!

In your question you said : >When the load factor of an aircraft increases above 1, the stall speed decreases. Well this is not true, saying that means that for example your stall speed in a 60 degrees turn will be less than the stall speed in straight and level flight. So it is obvious to say that when the load factor of an aircraft increases above 1, the stall speed increases

I would correct some words in the first paragraph : When the load factor of an aircraft increases above 1, the stall speed increases. As the load factor comes closer to 0, stall speed decreases and becomes infinitely low as an aircraft can not stall at 0G.

Yes, first two sentences are now correct after recent edit to fix another edit; stall speed increases as load factor increases, and is 0 at 0G.

It's never too late!

Only in aerobatic aircraft is the inverted stall speed identical to the upright stall speed in level flight. Airplanes registered in the Normal and Utility classes have cambered airfoils which have a higher positive maximum lift coefficient. In inverted flight, their stall speed is higher than their upright stall speed. The same goes for gliders and airliners.
When I had my last aborted winch launch in a ASK-13, I involuntarily checked how well the cockpit had been cleaned last time. I had to briefly close my eyes to avoid getting sand and dust into them. I can confirm that it is fully possible to fly a negative maneuver in upright position, albeit only briefly. However, the negative acceleration had as much to do with the rate of rotation and my position ahead of the cg as with negative lift.
If I had pushed a lot more, I could had stalled the wing by undershooting the negative lift minimum. But this would had just limited the downward acceleration, and the negative lift would not had been sufficient for inverted flight, because the inverted stall speed in level inverted flight was most likely higher than the speed at the time of the tow line rupture.
The negative stall speed is just your flight speed, and the possible negative gs grow with the square of speed. Stall is a matter of angle of attack, not of speed. A negative stall in upright attitude is rather uneventful - the aircraft will be at a high rate of rotation and will just hit the limit of downward acceleration.

Yeah, it is definitely a good way of checking the cleaning staff's work! Am I right to believe that as soon as you have a negative aoa, the lift will act as a downward force? And (I know this could be one on its own, but) does negative G Forces automatically mean a negative aoa? I think not...?

@Maverick283: No, the AoA must drop below the zero-lift value, which is slightly negative for most airfoils. But then, yes, the lift will act downwards. Also, the effect of pitch rate is considerable - a high nose-down rate might give the pilot the impression of negative gs while the wing is only around zero lift (and the whole aircraft around zero g).

Symmetrical aerofoils were also common in the early days of supersonic flight. So too were tailless aircraft, usually deltas. For example the Fairey Delta 2 speed-record-breaker had a symmetrical tailless delta wing, which had identical characteristics either way up.


text:
Do flight recorders need more efficient antennas?
At a frequency of 37.5 kHz, a quarter wavelength is about 2 km (1.24 miles) long. How does the black box efficiently radiate anything if it is not attached to 2 km of antenna?

The short version of the answer is 37.5KHz is not "radio frequency", it's "audio frequency", so an antenna is not necessary - just a (very loud) speaker, which is basically what the pinger is.

Specifically, the 37.5KHz "pinger" tones used in the Underwater Locator Beacons that are bolted onto Cockpit Voice Recorder / Flight Data Recorder "black boxes" are in the low end of what we call ultrasonic frequencies. Water is generally a good conductor of sound (with at least one notable exception), so it is usually possible for sonar detection equipment to "hear" the pinger tones at reasonable distances (with the interpretation of "reasonable" varying based on the depth of the area being searched - much like in air the sound intensity falls off as distance from the source increases, so if we're listening at the surface of the ocean a pinger in 500 feet of water will have a larger coverage footprint than the same pinger in 5000 feet of water).

Your question begs the equally-interesting radio-frequency question: "Aviation band radios operate in the frequency range 108MHz-137MHz (about 7ft to 9ft wavelength), so how can our aircraft radios efficiently radiate without being connected to 9 feet of antenna?" -

While the wavelength is 7-9 ft, the optimal antenna is (depending on antenna design) usually half, and sometimes a quarter (as mentioned in the question), so 2-4ft antenna would do the trick, which is, if I'm not mistaken, roughly the size used.

A quarter-wave antenna is easy to make: you've got the aluminum skin of the airplane to act as your reflecting plane, so it's just a matter of bracing two feet of wire so it's sticking straight out.

@falstro the point of that comment was to inspire someone to ask the question so we could give a proper answer with the quarter-wave dipole diagram and the RF theory (which will not fit in a comment) :-)


text:
What makes flight recorders harder to find than tagged sharks?
It really bothers me that we can't find planes that weigh tons with black boxes (AFAIK) technology that's almost obsolete, on the other hand shark tags (AFAIK) work great for long time and we can monitor shark movement, depth, temperature, etc., all the time. So, why?

Your question makes two unqualified assumptions - 1, that black boxes are often not found - in fact they are almost always are, and 2 - that shark tags are 100% reliable. I would suspect that some shark tags are lost, but this is not a big issue since many are tagged.

Yes, flight accidents are relatively rare and totally lost black boxes are more rare but it costs hundreds of lives and may be more if flight data that shows what had gone wrong is lost. so -black box aside- why flights depend on pilots communication for location and other flight information? what are the difficulties to send this data regularly and automatically during the flight?

Pfft, hundreds of lives? That's like e-9 size dent in human's genetic diversity, if that?

The "hundreds of lives" calculation assumes that the problem is one that can be prevented (to some degree) from recurring by obtaining knowledge of its occurence, and that such prevention will happen (it might be too expensive or politically unachievable). It also fails to consider the life analog of what economists call net present value: how far in the future would we expect to wait for the recurrence to claim lives if it is not prevented.

It should be noted that shark tags are as hard to detect as black boxes. They're only detected when the shark swims close to the surface. When the shark dives to deeper waters the sharks essentially disappears from our sensors. We get the depth record because the tag records them to memory and transmits them the next time the shark surfaces. So basically... black boxes would be as easy to detect as shark tags if we manage to teach crashed planes to swim like sharks (But then they'd be harder to get to because they keep moving around).

They're different usage cases, and have to be engineered differently. Three major points:

A shark tag doesn't have to survive a 500mph impact (#7 on the list) with the ground, and a flight recorder has to be able to do so. It adds a lot of padding and weight, and limits the kind of battery that can be used.

Sharks also don't dive to a depth much greater than 3000 feet on most occasions. A planes flight recorder is designed to survive to a much lower depth, under much larger amounts of pressure. Not to mention being designed to survive all sorts of other things.
Another point regarding depth, it is much harder to search for a signal at a depth of 15,000 ft, than it is even at a depth of 5000 ft. This article shows how big of a difference you get between even 20 meters and hundreds of meters (not thousands), it gives a good idea of what I mean.

If a shark tag fails, nobody cares. If you tag 100 sharks and 30 of the tags survive, you still have some pretty good data. But if one airplane crashes and its black box fails, it's considered unacceptable. They are required to survive 100% of the time.

All of these differences combined mean that black boxes are manufactured to much higher standards, and as a result they are bulkier, have different sorts of radios and require much more battery life (to support the larger systems) than a shark tag. All this conspires with the difficulty of getting a receiver deep into the ocean to make finding FDRs/CVRs really difficult.
That's not to say they couldn't design one that is much better, it just becomes extremely expensive (as all things are on airplanes). And you have to determine if these edge cases (like flight MH370) are worth making an airplane a few million dollars more expensive (between R&D and certification, the later being quite expensive).
I think the theory was they should be able to find any one of these within a month, and that may take some rethinking (maybe 3 months, or a year perhaps?) But they'll never be as successful as shark tags just because the requirements to be able to find one at that depth 100% of the time are quite different from the needs of a shark tag.

This answer sums up in a simple language why FDRs are designed this way, but, I was looking for one more point which is (why can't be better?) and I found it in the link you provided #8, thanks!

Moreover, found this fixes almost all the issues you mentioned in the old design.

Also, shark tags generally aren't required to survive being cooked in a pool of blazing jet fuel for hours on end.

Shark tags work by only sending the logged data when the shark resurfaces. Between those it just stores the data in memory until it can send it.
Sending underwater takes a lot of power otherwise.

According to this "Deployable Black Boxes" the technology exists, but the paperwork doesn't.

Deployable black boxes make it easier to recover the black boxes but not necessarily the plane. It would be the same situation as it was with the Titanic where we knew exactly where it sank but its resting place on the ocean floor turned out to be much harder to locate.

@slebetman The plane is gone, people aboard died, but FDRs with GPS, proper batteries and can resurface will be helpful to lessen future accidents

Because they're located in the wreckage, which can be at great depths. The only signal that can be sent at great depths is an audio signal, and that takes a lot of power (relatively speaking, the CVR and FDR are battery powered)
Sharks (or whales, they are often tagged) either remain in relatively shallow waters, or occasionally swim near the surface, where a radio signal can be detected.
The practical solution would be a realtime data feed of data now being recorded on the FDR to a remote receiver, sort of a super ACARS. In fact, ACARS data factored in determining the cause of the AF 447 crash long before the wreckage and black boxes were found - it showed a stall after loss of airspeed from frozen pitot tubes.


text:
Is this frost or frozen bacteria in clouds?
I was recently flying back from Palma de Mallorca Airport (PMI) to Birmingham Airport (BHX/EGBB) and I had the luxury of sitting at a window seat behind the wing. I think the plane was a Boeing 737-800 via Jet2.
Halfway though the flight I was looking out the window and noticed little specks on the outer window and was trying to work out if it was frozen water or frozen bacteria from the clouds?

Does anyone know what this is or how it forms? Does this happen to the pilot windows also?

What kind of bacteria is large enough to be observed with the naked eye? As far as I know there is only one species (Thiomargarita namibiensis), so what are the odds of this being this specific bacteria?

That's way too big for bacteria

I was thinking more like clumps or specs of frozen dust. Water made the most sense but I remember reading a white paper on how bacteria can travel in the cloud layer.

Are those specs really on the outside of the aircraft and not in between the outer and inner window panels?

@AEhere From what I could tell but my eyes can only do so much.

Is that coastline still Mallorca behind the trailing edge of the wing? I don’t have much comparisons, if it’s not frost I would assume usual cracks (looking there different): https://aviation.stackexchange.com/a/57359/1084. Question, I wonder about your usage of term „specs“ (here neither glasses, specifications nor speculation).

@peter The OP might have referred to ”species“ ;-)

@PerlDuck: Thanks! That makes sense :)

If it disappeared after you descended, it was frost specks on the inside surface of the structural* outer panel, forming on dirt particles on the plexi, from the small amount of humidity in the cabin air that migrates through the little vent hole in the inner panel that is part of the interior fascia.
It's not a problem on cockpit windows because they are heated.
*I say structural, because the outer pane of quarter inch thick acrylic has about the same tensile strength as the adjacent much thinner aluminum skin and is part of the pressure hull.

It is unclear from your photograph, but it can be either frost, dirt or surface damage on the plexiglass layer.
As far as I know there is only one species of bacteria visible with the naked eye (Thiomargarita namibiensis), so without a workable theory on how it got up to FL300+ in such numbers, that hypothesis is not plausible.

Apparently those bacteria live in the coastal seafloor off Namibia and are rather immobile, so any theory is promising to be somewhat entertaining. I believe that bacteria colonies could in theory be visible even for smaller bacteria, though. And also, I would be very surprised if it wasn’t simply frost.

It's not bacteria, but why would you assume he's talking about observing a single large bacterium instead of a colony?  Have you ever seen a petri dish? Bacteria are easily visible to the naked eye.

@JohnWiseman I have, in fact! And it was the fact that it looks nothing like a typical colony and lacks a growth medium that made the idea seem even more far fetched.

Just looks like crazing in the plexiglass to me.


text:
Why is the "Dutch" roll called so?
Why is the "Dutch" roll called so? When did this name enter common usage, and what is its origin? Please cite sources if possible!

According to Wikipedia:

"The origin of the name Dutch roll is uncertain. However, it is likely that this term, describing a lateral asymmetric motion of an airplane, was borrowed from a reference to similar-appearing motion in ice skating. In 1916, aeronautical engineer Jerome C. Hunsaker published the following quote: "Dutch roll – the third element in the [lateral] motion [of an airplane] is a yawing to the right and left, combined with rolling. The motion is oscillatory of period for 7 to 12 seconds, which may or may not be damped. The analogy to 'Dutch Roll' or 'Outer Edge' in ice skating is obvious." In 1916, Dutch Roll was the term used for skating repetitively to right and left (by analogy to the motion described for the aircraft) on the outer edge of one's skates. By 1916, the term had been imported from skating to aeronautical engineering, perhaps by Hunsaker himself. 1916 was only five years after G. H. Bryan did the first mathematics of lateral motion of aircraft in 1911."

Sadly, this question is probably not answerable beyond what wikipedia has on this site (unless there is an English Major looking for a Ph.D. thesis. ;)

This article has videos showing both an airplane and ice skaters doing the "Dutch Roll," as well as an explanation of the physics.

An early use of "Dutch rolls" in aeroplaning is 1910:
Fancy aeroplaning was on the programme at the Harvard Aviation Field the next day, with Ralph Johnstone and Walter Brookins as the star performers. They demonstrated their skill with the biplanes until Wilbur Wright feared for their safety and ordered them to desist. Johnstone's favorite stunt was to indulge in steep volplanes, shutting his engine almost completely off and  floating lazily for a moment on an even keel, then suddenly shooting down through space until het almost touched the ground. Then would follow a series of Dutch rolls as he swooped up and down close to earth.
Conquest of the Air by Airships and Other Flying Machines, Jay Henry Mowbray, 1910. (Link)

In Dutch, this motion is called zwierbeweging: swaying motion. Indeed the motion that can be observed during ice skating on modern metal skates.
I unfortunately have no official reference other than videos of for instance this event. Swaying Dutch roll everywhere.


text:
How are folding wings managed?
There exist several aircraft - especially carrier-based ones - that have folding wings, to save space.
How is this structure strong, taking into account the considerable amount of force on the hinge-based system (is it?) from the wing loading? How are they prevented from just snapping mid-air?

 Photo by Petty Officer 3rd Class Jason Johnston, U.S. Navy: Public Domain
The same question regarding helicopter rotors too.

 Photo by Lt. Col. Kevin Gross: Public Domain

Technically that's not a helicopter.

@Airsick: The mechanism and the component material was what I was asking about.

@CGCambell: Hehe. That's right! But this is used on other carrier-based helicopters too.

The wing folding mechanisms (in aircraft) and rotor folding mechanisms (in helicopters) are different. The V-22 Osprey falls somewhere in the middle.

In case of aircraft, the wing folding mechanism is usually made of high grade steel (or titanium in some cases) and actuated either hydraulically (or in recent cases) electrically. The outer wing rotates about a hinge, to which the two parts (inboard and outboard) of the wings are locked. For example, the F/A 18 wing folding mechanism looks something like this:

Image from US Patent Wing fold push-pin locking assembly US 5558299 A
In general, once the wings are either folded (or unfolded), they are locked using very hefty sliding pins which provide a secure mechanical connection once (they) slid into place in sockets in the outer wing.
However, incidents do happen, usually because the pins have not locked and not because of failure. A number of incidents have happened where aircraft have taken off with wings folded (and landed safely, surprisingly).

Source : reddit.com/theaviationist.com

In case of helicopters, the blade folding method is more or less the same with some variations- pull the pin that holds the blade (on one side) and fold the blades, which is done manually for small helicopters and hydraulically for large ones.

Source: reddit.com
They are then held either by hydraulic pressure or mechanical attachments.

Source: adf-messageboard.com.au
At-least in helicopters, there is little chance of a takeoff with the wings folded.
The V-22 Osprey is a special case- The propellers (or rotors) is folded first, then the engines are rotated, followed by the wings themselves.

Awesome answer.


text:
Why are all engines identical in typical commercial airplanes?
I come from the IT world where redundancy is in place to cover several cases, among other:

a statistical failure of a device (so we use more of them expecting that the event is independent)
a rogue usage of a device (hacking into it though a vulnerability)
a specific context where a bug is triggered

In order to account for the last two ones, redundancy is sometimes (or usually - depends on the criticality of the system) done with different devices (form different vendors). This is typically the case with firewalls, for instance.
I was wondering why this isn't the case with aircraft engines?
It would be "easy" with planes having four engines (so two of each kind), somehow less "easy" with two engines (where the differences between their regimes would need to be compensated - the pilot states that they want 80% of power, which, in the specific conditions they are in, means 78% for the left engine and 83% for the right one).
Having two redundant engines which are equally susceptible to be impacted by, say, ashes is probably not interesting, compared to having two different ones which would each have its strong and weak points (but different between the two)

Note 1: "easy" above is used as a placeholder only. I am aware that aircraft software and hardware is complicated, I mainly wanted to highlight the difference between 2 and 4 engines. Having two firewalls is a simple task compared to that.
Note 2: it may be that engine failure is not a big deal, statistically speaking compared to other issues (or in an absolute sense), and therefore the extra maintenance, structural compensations because of the differences etc. are not worth it. Thanks for pointing this out if this is the case

Possible duplicate of Why don't 747s use unreliable, but cheap, jet engines instead?

it's a KISS situation.

Possible duplicate of Are all the engines on a multi-engine plane the same?

It might work in 4-engine aircraft, where "inner" ones are engines X and "outer" are engine Y. I can't see putting different engined on both sides - you can't put different tires on both sides of a car, let alone engines on airliner. Now, most airliners have 2 engines...

@el.pescado Yes, you can

@Fattie I'd say the whole KISSATIT for this one, though...

Dissimilarity is often used for Avionics systems. For example an application specific integrated circuit will often be produced by two separate suppliers based on the same design.

Well, that would be a good start to create a labyrinthine system. As other stated, you would only make excessively complicated and complex. Additionally, you would raise maintenance costs by quite a bit.

On note 2: it may be that engine failure is not a big deal - that's true for four engines, but for two engines, it's not: The main point is to still have other engines left when one failed. That is, to have any engines left.

@StianYttervik I had to look up the KISSATIT principle - "Keep It Simple Stupid, And Think It Through!"- pronounced "kiss a tit" It's nice, but has some gender-asymmetry. KISSAASS would be more symmetric. KISSADICK has some gender-asymmetry also. I imagine to pronounce it at  a conference as "kiss a ... breast ".

Even for a two engine plane, one engine failure, after takeoff, is not a big deal. I don't know if the reliability figures include non-catastrophic failures, but pilots are well trained on engine shutdown procedures, they do make "precautionary" shutdowns, and after an orderly shutdown in the absence of catastrophic damage the aircraft handles perfectly fine.

You most likely wouldn't use different brands of HDD in a RAID setup.

...What? I've never heard of anyone specifically using different kinds of hardware to mitigate risk of failure in IT. Redundancy is nearly always exactly like you describe it being for jet engines: just having many of them.

@jpmc26 Funnily enough, aviation is a perfect example of such a situation in IT: flight control computers in some aircraft are intentionally made by different manufacturers using different parts to the same specs in order to minimize the chance of the same problem happening with all the redundant systems. They use triple redundancy and a 'voting' system. If one system disagrees with the other two, it is ignored, assumed to be faulty.

@jpmc26 as an example, using two different firewalls is the norm, the more distinct they autre, the better. It is more difficult for switches, but I had such a setup in a 5000 users environment. So now you have :)

I've never seen anybody worrying about hardware diversity in IT. It's always about software, because that's where orders of magnitudes more bugs happen. The reason you use different firewalls is because you assume there's a bug in its software, not because you assume that their hardware chips (which have a chance of being made by different factories even if they're the same product) will fail. Turbines are still pretty much purely mechanical and the software that drives them has indeed multiple redundancies.

@jpmc26 I have definitely seen people doing this for HDDs. Mixing HDDs from different production batches (from the same manufacturer) seemed more common, but sometimes with entirely different manufacturers.

In IT more hardware is redundancy, but in aviation a liability
Short answer

In IT, having redundancy and diversity is low cost and no liability, but high gains in service reliability.
In aviation, redundancy and diversity is high cost and a liability, for no increase in reliability of the service.

The long answer
The big difference is that in case of a hardware failure in IT, you just lose a bit of capacity. The service stays up, the customers do not even notice, you revenue stays intact. In IT, a failure is all right, and is allowed to happen.
In aviation, failure means you lose the entire service. All operations are centered on the philosophy that failure is not an option, in the sense: we can deal with it... but it costs a lot and we really do not want to do it.
Let us go through all the consequences of a failure...
Injury, loss of life

IT: a server blade goes down. So what? No-one is injured, no-one dies.
Aviation: An engine goes to pieces. That. Is. Not. Good. Losing/injuring colleagues and/or clients on the job must not happen.

Disruption to service, loss of revenue

IT: a server blade goes down. So what? The performance of the service is slightly degraded. But customers are not turned away. You do not lose revenue over it.
Aviation: an engine comes apart. You are now rapidly losing money, because as of that moment that aircraft is a cost, not a source of revenue.

The key difference here is that in IT you are only losing a percentage of the service. In aviation, as soon as an engine failure happens, you lose the entire service. This means more engines are a liability, because they mean more possible points of failure.
First the flight in question is instantly cancelled/interrupted, because you do not keep flying as if nothing happened. You are setting the aircraft down on the ground and you are doing it now. This means accommodating all the passengers, either hosting them in a hotel or rescheduling them on other flights... fares you have to pick up the tab for.
Second: the aircraft is now out of commission for unscheduled repairs. This is a big deal because airliners earn all their revenue while flying; they earn nothing while standing around in a repair-shop. An airliner spends about 2/3 of its service life in the air. That is to say: 16 out of 24 hours every day, for 20-30 years, an airliner is supposed to be in the air making money.
Third, you can bet your bottoms that someone had their phone up — even though you told them to turn it off — and filmed the whole thing happening. Then they call their tabloid and email over the link to their YouTube clip of it happening. Your airline's logo is now all over the evening news, your failure published nation- and world-wide. This means loss of customer confidence, which means more loss of revenue.
Replacing the busted hardware

IT: the cost of the broken hardware and the manhours needed to replace them are rarely counted in excess of 10 000 USD. Now granted this happens a whole lot more often than an aircraft engine failue, but in general, this is a very small cost.
Aviation: The cost of an engine is in the range of 5 000 000 USD and up. A new Rolls Royce Trent 1000 is 15 000 000 USD. Add to that hundreds of manhours to get the replacement engine out of storage, ship it to the location of the wounded bird, remove the busted engine, inspect the aircraft for damage and certifiy it fit enough to fly, put in the new engine, get the aircraft back in service.

Maintenance
Others have already gone through this so I will just mention it briefly: maintenace costs is counted per unit in aviation. Twice as many engines = twice as much maintenace cost. Such is not the case in IT.
Also — and this is the crux of your question — you ask why in aviation one goes for commonality instead of diversity. This is because in aviation knowledge and experience of a system are commodities. Double the number of types of systems (such as different engines), and you double the amount of people you need to hire, along with double the amount of experience you need to accrue.
Also the support infrastructure needed to deal with one type of system is different from the next. Again: you multiply the maintenance cost for every type of system you add to your organisation.
Conclusion
The only reason airliners have not gone down to using only one engine per aircraft is for safety reasons... since the only thing more unacceptable than having one engine quit in mid flight is to have all engines quit.
In IT, having redundancy is low cost and no liability, but offers high gains in service reliability. In aviation, redundancy is high cost and a liability, for no increase in availability of the service. The same goes for diversity: there are no gains to be had from it.
So in conclusion: failure is not an option. We can deal with it but — unlike in IT — any such failure is an expensive and very disruptive event. We just do not want to have to deal with it. Since more hardware, and diversity of hardware, increases both the risk of failure, and the maintenance and support costs for it, there is nothing to be gained from diversity and redundant redundancies.

Very nice answer.

"In aviation, failure means you lose the entire service." This is thankfully not true. Imagine losing all hydraulics because one system failed. Or all thrust because one engine failed. Or all comms because on radio failed. Or control of the aircraft because one flight computer failed. Aviation makes extremely extensive use of redundancy and quite a few of the major systems on passenger airliners are required by law to be redundant. Failures happen. They just do. While we do try to minimize them, we also make heavy use of redundancy to mitigate their effects.

As far as engine failures are concerned, they happen somewhat frequently. In the scope of a large airline, it's a relatively routine event (though thankfully not as routine as it was 40-60 years ago.) This possibility of failure is exactly why airliners are designed to be able to continue a take-off and climb-out with an engine failure and is also why airliners that flew long distances away from potential diversionary airports were required to have 3 or more engines in times past. Thankfully, reliability these days has improved to the point that 2 is enough, if stringent precautions are taken.

You don't hear about most airliner engine failures unless you specifically follow aviation news sources, though, precisely because they are relatively common occurrences. The vast majority of them are uneventful, resulting in a diversion and a delay, but no other damage to the aircraft or the people inside, thanks to the redundancies designed into modern airliners. You mostly only hear about the ones where the redundancies also failed - i.e. uncontained failures or failures of multiple engines.

@reirab "This is thankfully not true." Yes it is true. That flight is cancelled. Those clients do not get what they paid for. The service they requested suffers a major interruption. Regarding being able to survive the disruption, I make a special comment about that in the end: that has priority over everything else, that is not in contention. But it is an inconvenience to have to take that into consideration. Nothing you commented here contradicts that which I wrote: that an engine failure is a major disruption and — although survivable (in most cases) — bloody inconvenient.

"Those clients do not get what they paid for" In most cases, they do. Just not on that particular aircraft. Because major airlines have yet another level of redundancy: more planes. Even when referring only to that particular plane, there are tons of failures that don't result in losing the service of that aircraft. After all, you don't want your multi-hundred-million-dollar airframe sitting around longer than it has to. Major systems like engines will certainly ground the aircraft, but lots of other systems can fly inop precisely because of the redundancy in aviation.

@reirab "Major systems like engines " ...which is what the original question was all about. Read the headline again. So you are quibbling about things that are out of scope.

@MichaelK To perhaps be more clear, it is your answer's broad painting of redundancy as useless in aviation that prompted my objections. The particular type of redundancy suggested by the OP wouldn't be useful, but redundancy in general certainly is, both in regards to the engines and to many of the other systems. The reason the OP's suggested particular type of redundancy would not be useful is that it does not address a common failure mode, not because redundancy as a whole isn't beneficial.

@reirab My answer constantly refers to engines.

Let us continue this discussion in chat.

Engines don't fail, on average. And if they do, it's a very low probability that two engines will fail at the same time.
Modern jet engines are extremely reliable, with failure rates on the order of 0.01/1000hrs. And if you do have a failure, you have redundancy, as you have two (or more) engines. A plane can stay aloft and land with a single engine.
It would add a lot of complexity and maintenance costs, with no real benefits.
If the failure is external, such as water in the fuel, lack of fuel, or ash ingestion, two different engines are unlikely to help, because all jet engines works in a very similar fashion. The external problem would simply affect both, maybe at slightly different rates, but ultimately shutting them down anyway.
So it adds cost and complexity, without providing any real improvement. Jet liners simply does not crash because all engines stops due to internal problems.
The one case where it would help is if there's a design fault with the engine, e.g. manifesting in turbine breaking after x hours. But engines are extensively tested before being put into serial production, and this occurs very rarely on production aircraft. In addition, the likelihood of the same problem occurring on two engines in such a sport span of time as a flight, but not at all during testing, is a rather low likelihood.

They do fail - just rarely. Take the recent (April 2018) Southwest engine failure, the 2nd in a two year span (2016, then 2018), prompting action "...the FAA is now taking action. The agency announced it will issue an order in the next two weeks requiring ultrasonic inspections of some CFM56-7B engines to check for metal fatigue in fan blades."  http://time.com/5244808/southwest-airline-engine-failure-metal-fatigue/

@CrossRoads yeah, I see that I was a bit unclear. Updated it a little bit to make it more clear, thanks for the feedback :)

I understand that the probability of failure is very low. This covers the first point in my question. This does not help when both engines fail because of the same reason (ashes is what came to mind when I was thinking this out) - something which may be prevented by having different ones. But I understand all the other constraints.

Ashes will probably impact two dissimilar jet engines in a fairly similar manner, by abrasion and melting into a glassy substance due to the temperature. This is a property of how engines work, not a particular engine.

@WoJ you'd need 2 propulsion systems working on completely different principles to prevent a failure due to simultaneous ingestion of volcanic ash. And if a system were found that's as reliable and efficient as modern jet engines yet not affected by volcanic ash it's likely that it'd replace all jet engines relatively quickly in new designed aircraft.

Similar to ash, a flock of geese don't read the manufacturer's name and model number on the engine as they get sucked in, and even if they did would affect all turbofans in near enough the same way

@WoJ Respectfully, as someone who works on airplanes with different engine types amongst the same fleet.... OMG!?!?!  if airplanes had all of the things installed on them the lay public suggested, they would all be made of the same material as the black boxes. The collective flight data for an airline would be transmitting gigs of info to your servers via multiple broadband networks, all powered by the latest Intel processors running the latest version of Linux, calculating target N1 on the left engine & target EPR on the right just to prevent a situation that virtually never happens.

@Frank: I do not really understand your comment. You mean my question was not appropriate for this site? or not appropriate at all?

@WoJ No, I simply mean that this site gets a lot of suggestions from non-aviation folks about how to make aircraft better, without understanding that what they’re suggesting aren’t simple, practical or even feasible. From a maintenance perspective, what you’re suggesting would probably cause more problems than it would solve.

@Frank: I understand that, this is why I put so many disclaimers in my question. I get in my area plenty of questions as well - some better, some worse but rarely suggestions. I was not suggesting anything or asking a "how come you guys did not think about this obvious solution" question.

@WoJ Fair enough. I mistook it for another thread of the sort. I will post an answer shortly from a maintenance perspective.

A counterexample is British Airways Flight 38 which crashed due to a dual engine failure [strictly speaking a dual fuel delivery failure] caused by a design flaw in 2008.

Maintenance costs are a big deal. Maintenance costs across multiple engine types for a fleet of the same airplane would be a big deal - training, parts, etc.  Worldwide weather tracking & reporting lets pilots avoid ash, thunderstorms most of the time, and other severe weather which is rough to fly in and can leave customers rattled.
I don't think having an engine that would be strong in one thing and weak in another compared to the other engine would end up being the most fuel efficient scenario, and the airlines are all about fuel efficiency. Anything that raises costs will not fly, as it were, unless the gov't oversight agencies make it a mandatory requirement.

Without knowing I would guess blame allocation is also a factor: If you have 2 engines from 2 different suppliers and they somehow interact (think oscillations or similar), who is then to blame? Each supplier will try to shift the blame to the other supplier: "Our engine is fine. It is due to the other engine."

It could even cause problems if the same maintenance crews are certified for both classes of engine (in the name of efficiency) as following similar but not identical procedures can cause errors (things like wrong torques that aren't immediately obvious)

One thing not yet addressed is the failure mode of computer systems versus mechanical systems.
In computer systems, redundancy may not be enough if the failure modes are not independent of each other; that is, if there is a design flaw. If both systems contain the exact same bug, and perform the exact same calculations, both computers will have the same error. The solution is to have two independently programmed computers working on different hardware. This is done in aviation for the most critical flight computers.
Mechanical systems are much less likely to be exactly the same across two components. While a design flaw may lead to an eventual failure of a component, this is unlikely to happen across multiple components at the same time. Most extreme conditions are addressed during the initial testing, so most 'unnoticed' problems are gradual failures like fatigue. Since fatigue relies on random defects in the material, it is extremely unlikely that two systems fail at the same time.
To give a bit of an idea of how unlikely a double engine failure could be prevented by different engines, you can check Wikipedia's List of airline flights that required gliding, which I suppose is a reasonable compilation of dual engine failures. The vast majority is due to fuel exhaustion, shutting down the wrong engine, or extreme conditions that would have disabled any engine. While not contained in this list, I think the only accident I can think of that would have benefited from two different engines is British Airways Flight 38 which crashed short of the runway due to similarly clogged fuel/oil heat exchangers.

+1 This is the reason. This is not an issue of IT vs. aviation, but rather of software design vs. mechanical design. The probable failures modes are different, therefore the solutions are different. Software design doesn't really have a great analogue of mechanical fatigue. Software doesn't deteriorate with age. It either works under a given set of parameters or it doesn't.

@reirab - I'd have to disagree with you on one point: software does indeed rot - ask any software developer. Now, it may be that existing bugs are slowly found over time, but rot it is! ;)

@CrossRoads touches upon the upkeep implications nicely
Beyond that keeping engines all the same greatly simplifies operation and prevents potential user error. Keep in mind that many of the common airframes out there are of aging design (>20 years old) and generally come from a time when the mentality about computers was not what it is today. FADEC has made engine control on aircraft much simpler and potentially would make your situation possible. But in a time prior to that there are lots of parameters that would vary between engines, a pilot would be thus responsible for knowing double the amount of critical numbers just to fly the plane. For example

Fuel burn: Even similar engines on the same airframe will have different fuel burns. This is important for trip planning, efficiency, and reserve calculations. Generally in a multi engine aircraft you can make certain assumptions about all the engines being the same.
Thrust parameters: Back in the piston days this was manifold pressure in jets its N1, N1 and EGT. If you have one type of engine you should see pretty much the same numbers across the panel for a given power setting. If you introduce a situation where the engines are different you would need to know all the combinations for each independent engine. Quick panel scanning would also yield lots of confusion.
Engine operating parameters: Not all engines have the same operation parameters so you now create a situation where you must know the parameters of both engines and essentially the resulting set of operating parameters that will allow you to do something. Lets say engine 1 has a 5 minute max thrust limit and engine two has a 4 minute 30 second max thrust limitation you can now only do max thrust climbs for 4 minutes 30 seconds due to the lower number. You may also have differing emergency procedures which would make high stress situations even worse.
Connections: You also run into (on some level) a simple connection issue. Different engines may require different physical mountings as well as different electrical/control mountings meaning you are going to need some confusing mix of components.

Failure is a big deal but its just handled differently than you describe. In aviation the notion is you mitigate the engine failure by just having a second engine. There is not really an added benefit (in practice) by mixing up the engine types on an aircraft. Ignoring situations where the engines failed for external reasons (fuel run out, volcanic ash cloud etc) in the more recent engine failure incidents we have only seen a single engine fail even though both are of the same type.

FWIW some piston twins solve the counter rotating issue by having engines that actually rotate (at the crank) in opposite directions. While they are almost always the same engine design with cams and cranks made to spin in the opposing direction they are, strictly speaking, different engines.

However, engine intermix is often allowed, which causes noticeable changes in fuel burn, throttle stagger, and thrust parameters. For example, you can intermix CFM56 SAC/DAC on the A320. The DC-9/MD-80 allowed you to mix JT8D ratings and A/C variants.

Should "N1, N1 and EGT" be "N1, N2 and EGT" in the thrust parameters bullet point?

This answer is from a maintenance perspective:
Aircraft Maintenance seems so easy until the plane is loaded full of passengers and/or freight, there is a pushback tug hooked up, it’s raining or snowing and everyone is looking at YOU to do something quick to get the plane out on time. Why is this relevant?
Imagine you have two different engines installed on an airplane. The crew reports over the radio that the right engine has an issue, but the transmission was kind of garbled. Your lead/foreman dispatches you and lets you know that the right engine has something wrong. You get to the airplane and the crew says, “Yeah, my left engine widget thingy doesn’t work.” You say, “Ah. OK. I was told it was the right engine.” The crew says, “No, I stated on the radio the left engine was bad, the right engine DIDN’T have the problem.” OK.... No time to waste on a slight miscommunication, you go down to look at the left engine. You open it up, and based on some preliminary troubleshooting, you decide the “Widget Switch” is bad. No such part exists, but we will call it that.
You, being in a rush, call your lead/foreman and say, “Hey boss... Send me a widget switch. Oh by the way,” and just then someone from the ramp butts into your conversation thinking they are the most important person in the world, and asks, “Is the plane going to leave?” You respond, and then the pilot is standing over your shoulder wanting to know what you think it is. You explain all of this and then you go about your business removing the old part while your lead (who still thinks it’s for the right engine gets one on order for you).
The new part shows up, just as you have the old one removed. For all intents and purposes, both parts look identical, but one is capable of internally withstanding much higher pressures than the other. You pull it out of the box, and throw the new one in. Everything looks good and goes in great.
You run upstairs to do a quick engine run to make sure everything works. That’s all the manual says to do anyway. You fire up the motor, but because there are two different motor types installed, there are different EGT limits, oil pressure limits, and fuel flow limits. There are also different “widget” limits. In the rush, you fail to notice that your widget limits are different than those of the opposite engine. They are, after all just a few widget detection units different from one another.
You decide it’s good, and sign the logbook off. You depart the airplane, get back to the shop, and begin the process of starting your paperwork. As you’re processing the part, you notice that the part numbers don’t match. Just then you hear your jet roar overhead on takeoff. You get that sinking feeling in your stomach. After 5-10 minutes, another call comes over the radio. The plane you just “fixed” is turning around with one engine shutdown.
The investigation is concluded with you as the sole proprietor of the blame because you failed to check you were putting on the right part. Maybe that’s true, but when all the holes in the Swiss Cheese line up, this is exactly what happens. Bad weather, garbled radio transmissions, interruptions, everyone believes in a sterile cockpit environment but no one seems to believe in a sterile maintenance environment, confusion of parts interchangeability, etc... They all play against you.

In the IT world, redundant hardware is typically on standby to be used as failover. This comparison does not fly [sic] when it comes to airplanes.

The engines on an airplane are in continuous use all the time, so no redundancy there. A plane cannot lift off with just one engine.
Like you stated you want similar performance between left and right engines, which would need to be compensated all the time if the engines were of a different type from a different vendor.
You actually have double the risk to have a grounded airplane due to unforeseen problems with a certain engine type (think of airworthiness directives)
There also is the maintenance issue that CrossRoads highlighted. You'll need spare parts and trained technicians for both engine types, which will be much more complicated and costly.

And another point: who says they're identical :) While a lot of parts are, many aircraft have slight differences between engines, mostly related to mounting and where intake and exhaust pipes and valves are located.

A plane cannot lift off with just one engine? :/...

@Cloud well technically some of them probably could but lets say its hardly practical ;-)

"A plane cannot lift off with just one engine." Yes, it can. This is actually a certification requirement for all modern twin-engine airliners. If it can't do that, you can't fly it in scheduled passenger service. Otherwise, routine bird strikes would be fatal (to more than just the bird.) You're right, though, that airliners don't have failover engines. They do operate with (greatly) reduced performance when flying with a failed engine, but it's still enough performance to complete a take-off run and fly.

The technical term for this problem is common-mode failure, and it's a central focus of modern aircraft design.  Where safety is assured by the use of redundancy, common-mode failures are a threat to the effectiveness of that redundancy, so are eliminated or mitigated at every turn.
When it comes to engines, common-mode failures virtually always originate from outside the engine itself.
All engines attached to the same aircraft will generally fly through the same environmental hazards (ash, wildlife) at the same time and are liable to the same modes of failure as a result.  Using different engine types would only enhance safety if one of those types was shown to be less susceptible to those failure modes than the other - but in that case, why not make them all of that type in the first place?
All engines are fed by the same set of fuel tanks, so if they all run dry (which is usually due to pilot error rather than mechanical failure), they'll all flame out at nearly the same time - unless the engines are so different as to require different types of fuel.  With that said, in practice the two sides of the aircraft operate independent fuel systems under normal circumstances, so that a leak that goes unnoticed for a while only has a chance of draining half of the fuel tanks and disabling half of the engines.  This is the type of mitigation usually applied to common-mode failures.
Aircraft have existed with dissimilar engines, however.
The most obvious and famous example would be late versions of the Convair B-36 Peacemaker, which had six "pusher" propellors and four turbojets, hoping to obtain the performance benefits of both types rather than reliability:

Beginning with the B-36D, Convair added a pair of General Electric J47-19 jet engines suspended near the end of each wing; these were also retrofitted to all extant B-36Bs. [...] The jet pods greatly improved takeoff performance and dash speed over the target. In normal cruising flight, the jet engines were shut down to conserve fuel. When the jet engines were shut down, louvers closed off the front of the pods to reduce drag and to prevent ingestion of sand and dirt.
As engine fires occurred with the B-36's radial engines, some crews humorously changed the aircraft's slogan from "six turning, four burning" into "two turning, two burning, two smoking, two choking, and two more unaccounted for."  This problem was exacerbated by the propellers' pusher configuration, which increased carburetor icing. [...] Three engine fires of this nature led to the first loss of an American nuclear weapon when a B-36 crashed in February 1950.

As an experiment, a VC-10 airliner in military service had one pair of its original turbojet engines replaced with a single, much larger turbofan.  The object of the exercise was to assist with the flight test program of the new engine.  Reportedly, the VC-10 flew well in this configuration, but was subsequently found to have suffered severe airframe distortion due to the asymmetric thrust loads, and was retired from service forthwith.
The de Havilland Hornet, essentially a heavy-fighter derivative of the better-known Mosquito light bomber, was fitted with a pair of Rolls-Royce Merlin engines - but with two different Mark numbers (130, 131).  In almost all respects these engines were identical, except for the final-drive gearbox; one of these reversed the direction of drive to the propellor, while the other did not.  This made the aircraft equally easy to turn to either side (much more important in a fighter than a bomber), and also made it more forgiving to fly in an engine-out situation.
Arguably, aircraft in push-pull configuration, such as the Cessna Skymaster, could be described as having this property.  Although both engines are nominally similar, their mountings are sufficiently different as to induce different failure modes under different adverse circumstances.  Nevertheless, the individual failure rates per engine will be comparable with conventional single- and twin-engine aircraft, and any engine failure will be treated as an emergency requiring an expeditious return to solid ground.

The winner example would probably be Rutan's Boomerang :)

@Zeus The Boomerang uses two very similar engines, so it's really just an extension of the push-pull concept, in the context of this question.

All commercial airplanes have at least two engine types. This is true even for single engine aircraft.
The first engine type is of course whatever the airplane uses. The second engine is gravity.
As part of passing airworthiness tests, all commercial airplanes must be aerodynamically stable. For an airplane to be aerodynamically stable it must have the property of having a positive static margin. Static margin is one aerodynamic measure of an airplane's stability.
In layman's term, static margin is often called "can glide". Airplane that cannot glide are simply not stable enough to be controlled by a human. Note that airplanes that cannot glide can be controlled by computers. In fact a lot of modern jet fighters are deliberately designed to be unstable (cannot glide) in order to improve areal combat performance.
Because of this, there is nothing to be gained by using different engines on an aircraft while incurring significant disadvantages in terms of engineering, maintenance, drag and fuel management.

Note: for a reference on how far commercial airliners can glide, google "Gimli glider". A typical airliner is almost as efficient as a purpose-designed glider or sailplane.
Also note: I used the word "airplane" instead of "aircraft" because this does not apply to helicopters or (in the near future) multirotors. For helicopters there is a similar reasoning due to autorotation. For multirotors most designs cannot survive total power lost so will just drop like a rock. Some designs are based on helicopter collective pitch so they can also do autorotation

At 200ft after takeoff, your second 'engine' type is really mounted the wrong way round. This does not actually answer the question.

This principle of safety through using different designs for the same goal is actually used in real planes, just not for the engines.
For some of the most critical system in a plane (like for example everything involved in making the flaps move when the pilot decides so), there is often a fully electronic system supplemented with a mechanical fallback. Redundancy is just one of multiple ways available to plane and systems manufacturers to prove that their plane is safe to the certification authorities. The norms currently used in most of the world actually define quite precisely what is expected when two systems are redundant. One such expectation is that the two systems come from two different companies and that the engineers didn't meet for lunch (I'm paraphrasing). The point is that common modes of failure caused by design defects are avoided by ensuring that the engineering of the two systems was done independently.
Safety is all about tradeoffs and experience (and sometimes, the traditions of the authority...). Not that many companies make jet engines... I can guess that engine problems almost always occur due to the environment (and thus would impact all engines in the same way) and not a fault in the way the engine was made. Engines are tested with incredible rigor. Considering all this and the logistic/piloting problems that using dissimilar engines would bring, there is little point in not using the same engines.

Many engine failures are caused by defects present at manufacture (such as imperfections in fan blades) or by improper maintenance procedures (which are very specific to the engine type).


text:
How do fuel trim and air bubbles prevention work in inverted flight?
I was reading Why do many GA aircraft not have a "Both" fuel selector? and voretaq7's excellent answer got me to thinking.
He cites the regulations noting that the plane must only draw fuel from one tank or have a method for preventing air from entering the fuel lines. However all his examples assume gravity in a somewhat constant direction.
What do you do when this is not the case? For example in a barrel roll do you set the fuel selector for the wing going up first? Does it depend on the plane? Do planes in air shows have special fuel systems?
How does this regulation apply to aerobatic aircraft, and how is the regulation satisfied for such aircraft?

As long as it's a positive G maneuver, it isn't a problem. Barrel rolls aren't necessarily negative G maneuvers, in fact when done correctly they are +1G through the entire roll. Planes that can fly negative G maneuvers usually have an inverted fuel and oil system.

Related questions on fuel systems for acrobatics, inverted fuel systems in fighter jets and why inverted flight may still starve the engine of fuel.

I've read those questions, but they do not address the air bubble regulation.

A typical GA airplane will draw fuel from at or near the bottom of the tank.
An aerobatic airplane will have a hose from the engine to the middle of the tank, then on the inside of the tank there will be another flexible hose with a weight on the end.  This inside hose is called a "flop tube".
When the plane is in upright flight, the "clunk weight" on the bottom of the "flop tube" (these are the real terms for them, I'm not making this up!) will rest on the bottom of the tank and draw fuel from there.
When the plane is inverted, the weight will move (along with the fuel) to what was formerly the top of the tank.
This ensures that as long as the tank has a reasonable amount of fuel in it, the clunk weight will be submerged most of the time.  There are occasions (usually rolls while going straight up or down) where the weight (and end of the flop tube) is momentarily exposed to air in the tank.  The engine will "hiccup", but there's usually enough inertia in the propeller and engine parts to restart the engine by itself when fuel flow is restored a fraction of a second later.
There's an additional component - a special header tank - that helps by confining the flop tube to a very small tank that's 100% full of fuel 99% of the time.

ok, so the fuel draws fine. What about preventing air?

I've edited the answer.  Details on how aerobatic header tanks work should probably go into a different question, lest this answer become too long.


text:
Why does the DC-9 have engines that are angled slightly upwards?
As per subject, I see that the above mentioned aircraft and some others that have engines mounted on the rear of the fuselage, have the engine intakes tilted slightly upwards. Why has this solution been implemented?
Wouldn't it be better having them parallel to ground?
If the airflow of the exhaust is directed slightly towards the ground would't it affect the plane causing it to push the nose downwards?

Can you add picture showing what you mean?

I believe there this could be traced back to the so-called downwash angle of the air coming off the wing.
Looking at the picture below (and put on some imaginary engines on the back like the DC9/MD80), it becomes clear that by mounting it upwards by a few degrees would make it meet the airstream at the streamline angle:

You would want to that streamline to interact with the aircraft fan blades with the wind at the same angles. I would imagine that if you did not do this, it would result in an uneven force on the fan blades, which would probably be highly undesirable.
Looking at the drawings of the DC9, it might appear that the shorter version has a higher angle, and that the longer version has a smaller angle, as further from the wing the downwash angle is less, as could also be seen above.

For a little further analysis, the following is apparently off the DC-9 Manual, and might shed a little more light on the design, in particular in the last paragraph:

"The nacelles have been designed to enclose the JT8D engine with a
  shape that will allow the highest installed performance ( thrust minus
  drag ) possible at all flight conditions. The nacelle nose cowling is
  nonsymmetrical to keep the drag to a minimum at the high mach number
  cruising conditions.
The nonsymmetry is required to maintain an extremely flat nacelle line
  adjacent to the fuselage, such that the nacelle supervelocity
  increments, when added to those from the pylon and fuselage, do not
  lead to super-critical velocities with resultant shockwave losses.
The nacelle axis and pylon have been set at 3 degrees angle of attack
  relative to the fuselage to align them with the local flow conditions
  prevalent at cruising conditions. This is necessary to eliminate the
  interference and high induced drag that they would otherwise be
  subjected to. The nozzle is then canted upward relative to the engine
  axis, thus eliminating the pitching effects due to thrust."

Following picture appears to show the nozzle angle is in fact more horizontal than the entry angle, with the underside longer than the upper side:

Thus, while it might not aesthetically pleasing maybe to have it off by a few degrees from the fuselage body, but it does bring advantages when seen with the airflow, and resulting concerns about that the thrust is not being vectored along the body axis is corrected by the nozzle.

Amazing answer! That really cleared up everything. Thanks!


text:
Is the ATR empennage configuration considered a T-tail or a cruciform tail?
How is the ATR 42/72 empennage considered? Is it a T-Tail or a cruciform tail?
From the image the elevators are placed quite high on the tail that they can be considered as a T-Tail but is it so?
Looking at the tail of a 727 the elevators are placed at the top of the tail but just leaving the space for the mechanism to move them, whereas on the ATR they are placed in a slightly lower position.
Are there any technical specifications and measurements that clearly define the difference between the two mentioned configurations?
I understand that cruciform means "similar to a cross" but it would be interesting to know, as previously mentioned, if there are specifics about this definition making it clear for everyone from a technical point of view. (briefly, when does a T-Tail become cruciform?)

Why do you think there should be clear boundary? Those are just common names for the designs. And there is no principal difference between them. All parameters vary continuously with the position of the horizontal stabilizer.

If we were in any other industry I would have closed an eye on it but since we are talking about aviation where everything is standardized and everything is documented I thought that such differentiation would be in some way distinguished. That's all

As far as technical specifications are concerned, some explanation is:
Excerpt from Wikipedia (formatting is mine):

T-tail
DESIGN: The horizontal stabilizer is mounted on top of the fin,
  creating a "T" shape when viewed from the front.
PURPOSE:

T-tails keep the stabilizers out of the engine wake, and give better pitch control.
T-tails have a good glide ratio, and are more efficient on low speed aircraft.
However, T-tails are more likely to enter a deep stall, and are more difficult to recover from a spin.

T-tails must be stronger, and therefore heavier than conventional
  tails. T-tails also have a larger radar cross section.
Cruciform tail
DESIGN: The horizontal stabilizers are placed midway up the vertical
  stabilizer, giving the appearance of a cross when viewed from the
  front.
PURPOSE: Cruciform tails are often used to keep the horizontal
  stabilizers out of the engine wake, while avoiding many of the
  disadvantages of a T-tail.

In reference to your comparison between ATR 42/72 with Boeing 727, if we take out the difference in tail heights (ATR 42: 24' 11"; ATR 72: 25' 1"; B727: 34' 0"), the position of horizontal stabilizer on vertical stabilizer is almost the same on these three airplanes. So they all match closely to a T-Tail configuration rather than a cruciform.

authors: Χριστίνα Σκρέκα